1. Field of the Invention
The present invention relates to a display device and an electronic device using the display device. In particular, the present invention relates to a display device using a thin film transistor in a pixel portion and an electronic device using the display device.
2. Description of the Related Art
In recent years, techniques that are used to form thin film transistors using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate with an insulating surface have been put into practice in many electronic devices. In particular, thin film transistors have been put into practical use as switching elements in a pixel portion of a display device, and research and development has been actively conducted.
As a switching element of a liquid crystal display device, a thin film transistor using an amorphous semiconductor film in a large panel or a thin film transistor using a polycrystalline semiconductor film in a small panel is used. As a method for forming a polycrystalline semiconductor film, there is known a technique in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system, and an amorphous semiconductor film is crystallized by being irradiated while being scanned with the linear laser beam.
As a switching element of an image display device, a thin film transistor using a microcrystalline semiconductor film is used (Reference 1: Japanese Published Patent Application No. H4-242724, Reference 2: Japanese Published Patent Application No. 2005-49832, and Reference 3: U.S. Pat. No. 5,591,987). In addition, as a method for manufacturing a thin film transistor for improvement of characteristics of an amorphous semiconductor film, a method is known in which an amorphous silicon film is formed over a gate insulating film, and then a metal film is formed thereover, and the metal film is irradiated with a diode laser beam to modify the amorphous silicon film into a microcrystalline silicon film (Non-Patent Document 1: Toshiaki Arai et al., SID 07 digest, 2007, pp. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is provided to convert light energy of the diode laser beam into thermal energy and should be removed in a later step to complete a thin film transistor. That is, the method is that in which the amorphous silicon film is heated only by conduction heating from the metal film, thereby forming a microcrystalline silicon film that is a microcrystalline semiconductor film.